In the more particular case of the intermediate axis, the latter has a Cardan joint at each end: one Cardan joint is connected to the motion input of the steering gear case, and the other Cardan joint is connected to the upper column portion. In order to have correct steering of current automotive vehicles, it is required that the length of the intermediate axis be able to vary and adapt to the oscillations of the front gear of the vehicle, due to the profile and the surface condition of the road carpet. This characteristic is also required in order to make the assembling to the rack pinion easier, and to absorb motions upon a frontal impact of the vehicle.
The intermediate axis should therefore have first a variable length, that is it should have a sliding function of two shafts with respect to each other along the column axis thereof, which is the intermediate portion axis. Moreover, a transmission function for the rotation movement between the two shafts and for the torque necessary for handling the steering is required.
There are numerous sliding shaft coupling devices, which enable to combine the passing of the torque between the two shafts through splines which are provided on each of the two shafts with conjugated profiles. However, this type of device has a clearance after an endurance cycle equivalent to the service life of a vehicle, which is the one required for current cars. In order to delay the onset of such clearance, adjusting the sliding of both shafts upon manufacture is relatively tight, which requires upon the line assembly a quite high axial stress, hence a longer assembly time and more laboriousness upon such mounting.
Such adjustment should be accurate in order to enable operational axial movements to be correctly absorbed. Except upon the passing of the torque, the axial stress is a function of the torque being transmitted, the sliding stress related to the torque being transmitted and the friction coefficient increases, and then there is a sudden axial release, causing jerks which prevent a good sliding adjustment with reduced clearance to be maintained; and being harmful for a good driving feeling.
There are also coupling devices which use plastic injection on splined portions which are part of a male metal shaft and a female metal tube. This solution raises issues for absorbing axial movements under high torques; the sliding stresses increase proportionally to the friction forces between the two portions. Besides, the plastic injection wear leaves clearances. As a last solution, rolling elements and strain springs can be introduced between the shaft and the tube. This satisfying solution for a smooth sliding raises issues of angular rigidity because it is directly proportional to the stiffness and preloading of the springs.